1. Field of the Invention
The present invention relates to a motor control device which controls a motor and a motor drive system.
2. Description of Related Art
When vector control is performed for a permanent magnet synchronous motor, a control axis to be rotated in synchronization with rotation of the motor is defined in a motor control device, so that current information and voltage information are controlled on the control axis for realizing desired vector control.
When the motor rotates at high speed, flux-weakening control (field-weakening control) is usually used so as to suppress an excessive increase in an induction voltage (i.e., electromotive force) that is generated in the motor due to the magnetic flux of a permanent magnet and the high speed rotation. Under the general flux-weakening control, on the assumption that there is no difference between the control axes and dq axes, d-axis current (field current) that is a field current component of current supplied to the motor is controlled on the basis of motor parameters. The motor parameters include a d-axis inductance of the motor, a q-axis inductance of the motor, an armature flux linkage by the permanent magnet, and the like.
It is required to realize stable flux-weakening control even if the control axes and the dq axes are shifted from each other, or if the motor parameters vary due to magnetic saturation or the like, or if the motor parameters include an error. In particular, if the motor is driven in an overmodulation region, it is required to control a voltage value applied to the motor to be precisely below a predetermined limit voltage value.
However, if a change or the like of the motor parameters occurs in the conventional system, too much or too little weakening magnetic flux is generated because of the change or the like, so that a desired flux-weakening control is hardly realized. According to a certain conventional method, it is possible to realize flux-weakening control that is not affected by a change in the q-axis inductance. However, this conventional method is not still sufficient as a measure against an error in the d-axis inductance and the armature flux linkage, or a change in the motor parameters due to magnetic saturation.
On the other hand, there is a method of generating a specified d-axis current value to be a target of the d-axis current (specified field current value) by using proportional-plus-integral control (hereinafter referred to as PI control). For instance, in a certain conventional method, the specified d-axis current value id* is calculated so that √{square root over ( )}(Vom2−vd*2)−vq* becomes zero, so as to control a magnitude of an actual specified voltage value (in other words, a magnitude of an actual specified voltage signal) √{square root over ( )}(vd*2+vq*2) to be a limit voltage value Vom or lower.
However, in the actual system, it is required that the specified d-axis current value id* does not exceed a predetermined limit value. Therefore, in the method of using the PI control, it is necessary to take a countermeasure of suppressing occurrence of a windup phenomenon when an output of the PI controller is limited by a limit value. This countermeasure may complicate a process algorithm of an integrator in the PI controller. The same problem may occur also in the case where the vector control to be performed is switched between the flux-weakening control and maximum torque control. The windup phenomenon is known as a phenomenon that an internal signal of the integrator in the PI controller diverges. Note that the configuration of using the PI control will be also described later as a comparison with a configuration according to the present invention.
The problem of the conventional method is described above by noting the specified d-axis current value related to the flux-weakening control, but other control values than the specified d-axis current value (specified value or state quantity indicating voltage, current, torque or speed) are also generated by using the PI controller in general. In addition, the control values are also required not to exceed a predetermined limit value. Therefore, also in the case where the PI controller is used for calculation of the control value other than the specified d-axis current value, it is necessary to take a countermeasure of suppressing occurrence of the a windup phenomenon. As a result, the process algorithm of the integrator in the PI controller is complicated.